1. Field of the Invention
This invention relates to nuclear techniques for the investigation of a material, in order to determine the characteristics of such material, such as e.g. its composition. To this end, the material is irradiated with neutrons which interact with atoms in the material and are then detected. The detection is followed by an appropriate process of the neutron counts to provide the required information related to the material under investigation. By way of non limiting example, the invention can be advantageously implemented in the nuclear well logging techniques, wherein a sonde is lowered in a well or borehole and the earth formations surrounding the well are irradiated with neutrons from a neutron source disposed in the sonde. Radiations resulting from the interaction between the formation atoms and the emitted neutrons are detected and processed in order to derive information on the composition and/or the physical structure of the earth formation, or the borehole fluid, or the annulus including casing and cement located between the borehole wall and the formation. The invention can also be applied to logging while drilling technique (usually referred to as "LWD") where measurement devices, disposed close to the bottom of a borehole drilling system, perform measurements while the borehole is drilled.
2. Related Art
Neutron detectors can be used in numerous instances wherein the detected neutrons are representative of a material under investigation. The invention will be hereafter depicted in the context of the nuclear logging techniques wherein the material under investigation includes the earth formation surrounding a borehole. A sonde, including a neutron source and at least one neutron detector, is lowered in the borehole, and one detects the irradiating neutrons after they collide several times with atoms of the formation. Neutron logs are generated from the detected neutrons. Neutron logs are used principally for delineation of porous formations and determination of their porosity. They respond primarily to the amount of hydrogen in the formation. Neutrons are electrically neutral particles, each having a mass almost identical to the mass of a hydrogen atom. High-energy (fast) neutrons are continuously emitted from a radioactive source in the sonde. These neutrons collide with nuclei of the formation materials in what may be thought of as elastic "billiard-ball" collisions. With each collision, the neutron loses some of its energy. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greater energy loss occurs when the neutron strikes a nucleus of practically equal mass, i.e., a hydrogen nucleus. Collisions with heavy nuclei do not slow the neutron very much. Thus, the slowing of neutrons depends largely on the amount of hydrogen in the formation. Within a few microseconds, the neutrons have been slowed by successive collisions to thermal velocities, corresponding to energies of around 0.025 eV. They then diffuse randomly, without losing more energy, until they are captured by the nuclei of atoms such as chlorine, hydrogen, or silicon. The capturing nucleus becomes intensely excited and emits a high-energy gamma ray of capture. Depending on the type of neutron tool, either these capture gamma rays or the neutrons themselves are detected and counted by a detector in the sonde. When the hydrogen concentration of the material surrounding the neutron source is large, most of the neutrons are slowed and captured within a short distance of the source. On the contrary, if the hydrogen concentration is small, the neutrons travel farther from the source before being captured. Accordingly, the countering rate at the detectors increases for decreased hydrogen concentration, and vice versa. Examples of such logging sonde are described in U.S. Pat. Nos. 3,509,343 to Locke, or 4,223,218 to Jacobson, or 4,926,,044 to Wraight, which are all assigned to the assignee of the present application, and which are incorporated herein by reference. Neutron detectors are generally of the so-called proportional type, containing ionizable gas, such as e.g. helium-3 (He.sub.3). He.sub.3 detectors are exemplified in U.S. Pat. Nos. 3,240,971 or 3,102,198, or in the article "Recent Improvements in Helium-3 Solid State Neutron Spectrometry", by T. R. Jeter and M. C. Kennison, IEEE Transactions on Nuclear Science, February 1967. vol. NS-14 No. 1, pages 422-427, or in the book from G. F. Knoll, "Radiation Detection and measurement", Second Edition, 1989, p. 494-496. All of the above mentioned documents are incorporated herein by reference.
The use of known neutron detectors, e.g. in the above mentioned nuclear loggings sondes, raises two problems.
Firstly, since a neutron detector has substantially no background counts, its emits no signal in the absence of the neutron source of the sonde. This results in a detrimental uncertainty as to the operation of the detector. Accordingly, it is highly desirable to be able to verify or check, before the sonde is lowered in the well, that the neutron detector actually and properly works.
As a matter of general interest, the word "verification" here refers to a checking step of the "go-nogo" type. The word "calibration" here refers to ascertaining that the detector response, outside the well is the same as the response established during shop calibration. Finally, "stabilize" and "stabilization" here refer to any step aiming at checking that the response of the detector, while in operation in the well, do not show any substantial modifications, such as e.g. offset or drift, which would be detrimental to the measurements.
The second problem encountered when using known neutron detectors relates to the calibration of the detector(s). Calibration, in the context of logging, consists of adjusting at the wellsite, prior to logging the well, the tool response to match that of the engineering reference established under laboratory conditions. This accounts for variation in detector sensitivity from tool to tool and with time. It also accounts for variation in source strength, which changes with time. What is known as the wellsite calibration is actually not a primary calibration, but rather a "verification", the purpose of which is to confirm that the tool is functioning and that its response has not changed since the last "shop" calibration which are carried out in designated area under predetermined conditions and using a specific set-up. This verification is typically done by monitoring tool response to a point source of .gamma. rays or neutrons strapped onto the tool before it is lowered in the well for logging. The article "Recommendations for Neutron Logging from the SPWLA Subcommittee for Log Calibration Guidelines", by R. Wiley and L. S. Allen, The log Analyst, May-June 1988, pages 204-214, provides general background on the known methods for calibrating, at the surface, neutron detectors disposed in a logging sonde. This article is incorporated herein by reference.
The above mentioned "surface" calibration methods suffer from various disadvantages.
They are based on the use of an external radioactive source which raises safety concern. Strict regulations have been and are still being enacted which set forth the steps to take in view of minimizing, if not avoiding, the risk of accident. Implementing these regulations make the logging operations more complex and time consuming.
Furthermore, the hardware or set-up used to perform surface calibrations is designed to allow easy transportation, at the cost of being sensitive to the surrounding environment. In order to minimize, if not avoid, the influence of the environment, surface calibrations have to be performed according to strict guidelines. Consequently, those calibrations are time consuming. This is detrimental in a business environment where time is highly priced.
Moreover, being carried outside the well, these surface calibration methods do not provide any assurance to the operator that the detector is actually and properly working during the logging of the well. For the same reason, the detector response they provide is not representative of the downhole conditions especially in term of temperature and pressure.
As an attempt to remedy this situation, it has been proposed to carry out verification "in-situ", i.e. in the well, shortly prior to and following the logging operations, as explained in the article "A New Calibration, Wellsite Verification, and Log Quality-Control System for Nuclear Tools", by J. R. Olesen, SPWLA 31.sup.rst Annual Logging Symposium, Jun. 24-27, 1990, Paper PP. The in-situ verification method is usually referred to as the "plateau" method. The plateau refers to a flat part of the curve of total count rate versus detector high voltage. The detector voltage is adjusted so that the operating point is at the middle of the plateau, thereby ensuring constant detector sensitivity regardless of variation in temperature. The plateau test shows whether the tool is working in its middle range, the optimal operating point, rather than near the end. If the tool is working in the middle range of the plateau, this verifies that analog parts of the tool system have not drifted. Before logging, this verification is done typically a few hundred feet below surface where the tool is still at quasi-surface temperature and pressure. After logging, the verification is repeated typically just above the casing shoe, where the tool remains at logging temperature and pressure. The main advantage of the in-situ method compared with the surface method is that it is performed at in-situ temperature and pressure, and it is claimed to verify tool performance under operating conditions. For detectors showing the "plateau" phenomenon, the verifications carried out shortly before and after logging usually involve signals generated 1) by a special wellsite verification device, or 2) by the wellbore and logging source (as described in the above mentioned article by J. R. Olesen). The data obtained during these verifications are compared to similar data obtained at the time of the shop calibration. This verifies that, before and just after logging, the hardware was set at a stable operating condition similar to shop calibration conditions.
However, this "in-situ" verification method, although substantially improving over "surface" verification methods, does not overcome all the drawbacks of the latter. Particularly, the "in-situ" method is not performed in real time but actually is a verification "after the facts".
In view of the above, there is a strong need for a method which would provide the user with a reliable answer as to whether a neutron detector, actually and properly works. This need is all the more felt in the logging techniques where it is highly desirable to carry out the checking before the sonde, including the detector, is lowered in the well.
Moreover, beside the above referred to verification, it is highly desirable to stabilize, especially in real time, the actual response (usually a spectrum) of a neutron detector operating in a well.